Large High Altitude Air Shower Observatory

The Large High Altitude Air Shower Observatory (LHAASO) mainly aims at exploring the origin of high-energy cosmic rays and conducting scientific researches on high energy astrophysical radiation. LHAASO is located 4410 meters above sea level (a.s.l.) on Mt.Haizi in Daocheng County, Sichuan Province, China, and  covers  an  area of 1.36   km². The construction of LHAASO project is under the charge of the Chinese Academy of Sciences (CAS), executed by Institute of High Energy Physics and the Chengdu Branch of CAS. The construction of LHAASO lasted for 4 years and was completed in July 2021, then the full array was put into operation. The project passed the national acceptance on May 10,2023. LHAASO is currently operated and maintained by the Institute of High Energy Physics, CAS.

LHAASO consists of three arrays: a ground particle detector array made up of 5216 electromagnetic particle detectors (EDs) and 1188 muon detectors (MDs), covering an area of 1 km²; a water Cherenkov detector array (WCDA) consisting of 3120 detector cells, covering an area of 78,000 m²; and a telescope array (WFCTA)composed of 18 wide-field-of-view Cherenkov telescopes.  By employing hybrid measurements of extensive air showers (EAS) through these detector arrays, LHAASO achieves unparalleled sensitivity in detecting ultra-high-energy gamma rays and conducting all-sky surveys for very high-energy gamma ray sources. Additionally, it will provide measurements of the energy spectrum of cosmic rays across an exceptionally broad energy range. LHAASO has become a unique, comprehensive, and open science platform.

COSMIC RAYS

◎ What are cosmic rays?

Cosmic rays are high-energy particles coming from outer space and are mainly composed of protons and the atomic nuclei of many elements, as well as a small fraction of electrons and Gamma ray photons. Constantly present on our planet, cosmic rays were first discovered by Austrian scientist Victor Hess in 1912.

Cosmic rays span an energy range of 10⁹-10²⁰ eV. As the energy of cosmic rays increases, their abundance decreases, so larger area instrument for their detection are required.

The origin of cosmic rays -A mystery of the century

The majority of cosmic rays are charged particles, which are deflected in the interstellar magnetic field during their propagation. When they arrive at the earth, the original directional information is lost. We are yet to solve the origins of high-energy cosmic rays, thus to find out the mechanism of their production and acceleration.

Cosmic rays carry important information of the nature, including the evolution of the universe, celestial dynamics, solar activity and geospatial environment. The origin of cosmic rays has been listed as one of the top scientific questions in astrophysics.

DETECTION OF HIGH-ENERGY COSMIC RAYS

When high energy cosmic rays enter the atmosphere, they interact with atomic nuclei in the   atmosphere in a cascade mode and generate air showers consisting of enormous secondary   particles, called the Extensive Air shower (EAS). The shower particles travel almost in the speed  of light, with a thickness less than 100 nanoseconds when measured in time, and can spread across an area of several square kilometers in the ground.

ED: Measuring the secondary electromagnetic particles in the EAS. The scintillation light generated   by charged particles in the plastic scintillator is collected by wavelength-shift fibers, conducted     to a photomultiplier tube (PMT)and converted into electrical signals. The effective area of each ED is 1 m² .

MD: Measuring the muon content of the EAS.A water bag containing ultra-pure water is placed in a concrete tank with a diameter of 6.8 meters and a height of 1.2 meters, and a PMT is installed at the top center of the water bag to collect the Cherenkov light generated in the water by muons that enter the tank.

WCDA: Measuring charged particles and photons in the EAS. It is composed of 3 adjacent large ponds (two with a dimension 150 m×150 m, and one with a dimension of 300 m×110 m),and the water depth is 4.5 m. Ponds are sub-divided into 3120 cells in size of5  m×5 m, each provided   with 2 PMTs for collecting the Cherenkov light generated in the water by charged particles in the  EAS.

WFCTA: Measuring the Cherenkov lights generated in the air by EAS. Each telescope consists of a reflector made of 5 m²spherical aluminized mirror and a silicon photo multipliers (SiPMs) camera with 32×32 pixels. The field of view of a telescope is 16°×16°, and the pixel size of the camera is 0.5°×0.5°.

SiPM camera: Silicon PMTs were adopted in the WFCTA. This changed the traditional   observation restriction that such telescopes can not work during the lunar night. As a result, the   effective observation time was doubled.

New technologies

Time synchronization system: Based on “White Rabbit” technology, a large-area, multi-node, high-precision clock synchronization technology adapted to wild field conditions at high altitudes above 4000 meters was developed. This technology improves long-distance synchronization accuracy to 0.2 nanoseconds.

Large size PMT: A type of 20-inchulra-large photomultiplier tube was developed in China. It improved its temporal response three-fold, greatly expanding the observation capacity by lowering the detection threshold from 300 billion to around 70 billion electron volts for WCDA.

DAQ system: A big-data acquisition technology has been achieved, enabling “triggerless” data acquisition and “zero deadtime” observation to cosmic ray events with a data transmission rate up to4GB/s.

Data Transmission system: A special noise filtering technology and lossless compression algorithm has been developed and adopted enabling real-time data transmission from mt Haizi to IHEP.

Highlight of Scientific Results

LHAASO has discovered a large number of ultra-high-energy cosmic accelerators in the Milky Way, and recorded the highest energy photons, initiating the era of ultra-high-energy gamma-ray      astronomy (Nature,594,33-36 (2021),3 June 2021).

Precisely measured the high-energy luminosity of the standard candle Crab Nebula, detected photons beyond 1PeV, approaching the theoretical limit (Science,373,425-430 (2021),23 July 2021).

Detected the brightest gamma-ray burst event GRB 221009A, achieved the first complete observation of the afterglow of a GRB above 100 GeV, revealed the rapid rising phenomenon of the afterglow for the first time and discovered the rapid decay phenomenon in the afterglow of GRB 221009A(Science,380,1390-1396 (2023),8 June 2023).

Verified the correctness of the space-time symmetry in Einstein’s theory of relativity with the strictest examination (Physics ReviewLetters,128,051102 (2022),3 February 2022).

LHAASO’s measurements indicate that the dark matter particles, if their masses are around 1 PeV, would have a lifetime of at least a billion trillion years

International collaboration

Up to now, more than 280 scientists and students from 28 astrophysics research institutions have become members of the international collaboration of LHAASO. The collaboration explores the LHAASO data to conduct studies on astrophysics and other research fields.

LHAASO signed Memoranda of understanding (MoUs) with 10 collaborations, including seven international experiments (VERITAS, MAGIC, LST/CTAO, eROSITA-DE, ANTARES, Baikal-GVD, KM³NeT) and three domestic experiments (MWISP, FAST-CRAFTS, DAMPE), to perform joint observations. By this, LHAASO has established the cooperation with most of gamma ray astronomy experiments and neutrino astronomy experiments in the world, to build up a network on the multi-wavelength and multi-messenger astronomy.